German Patent DE 32 19 503 discloses an apparatus for automatic focusing onto specimens to be viewed in optical devices. After reflection at the surface of a specimen, the reflected measurement light beam bundle passes through a pinhole after reflection at a splitter mirror. A portion of the measurement light beam bundle is reflected out by means of a fully mirror-coated surface, and after passing through a slit aperture is directed onto a differential diode. In the focused state, the focus is located between the two diodes. In the event of defocusing, the measurement spot drifts onto one of the two diodes, which are connected to corresponding control means. The control means adjust optical or mechanical means of the microscope in order to bring the measurement spot back between the two diodes, and thus reestablish the focus position.
European Patent Application EP-A-0 124 241 discloses a microscope having an automatic focusing device. The microscope encompasses a memory device for saving the data from the objectives that are used in the microscope. Also provided is a control device which monitors and regulates the various microscope functions. Another of the tasks of the control device is to move the focusing stage. A CCD element, which receives an image from the particular selected objective and, together with a computation unit, determines the image sharpness based on optimum contrast, is provided as the image acquisition device. The objective data of the objective presently in use must be taken into account when determining the optimum sharpness. Those data are, as mentioned above, stored in a memory.
German Unexamined Application DE 41 33 788 furthermore discloses a method for autofocusing of microscopes, and an autofocus system for microscopes. The image of a specimen or of a pattern reflected onto the specimen is conveyed to two areas on a detector or to two different detectors; in the focused position, one image is produced in front of one detector, and one image behind the other detector. The image sharpness states on the detectors are converted into electronic signals, whose difference is used to focus the objective. The distances of the image or of the respective pattern from the respective detectors are adjustable. Deliberate offset settings, as well as “IR offset” correction settings, can be implemented.
A problem in the context of automatic focusing in microscopes, for the examination of specimens having several focal planes, is that the autofocus system does not know which plane it should sharply focus onto. The autofocus system can focus onto only one of the planes, and it can easily happen that the autofocus system loses the focal plane and jumps to another as it corrects. The multiple planes result, for example in the semiconductor industry, in the context of different topological steps or multiple photoresist layers. In conventional microscopy and in confocal microscopy, there are also multiple layers that can be focused on. In samples equipped with coverslips, these can be the upper side of the coverslip with the interface to air, or the underside of the coverslip with the interface to the sample.